The present invention relates to a method,
when using a single-grip harvester head for felling and processing trees, which head has a frame mounted on a forest machine and defines an essentially horizontal tree feeding path, two rotatable feeding rollers which are arranged on either side of said path and adapted to grip and to feed along the feeding path a stem, and at least one limbing knife assembly which is adapted to limb a stem which in an essentially horizontal position to which it is elevated by the head is fed by the feeding rollers along said path with its butt end first, the limbing knife assembly comprising an upper knife and two lower knives which are each pivotable about an axis, being essentially parallel to said path, in order to press, from below, by means of a controllable press power, the stem against a fixed support and together with the upper knife enclose the stem,
of controlling said press power,
in which method the diameter of the stem is measured during the feeding of the stem and the press power is reduced in accordance with a predetermined control diagram as the diameter decreases during the feeding,
the diameter of the stem being measured indirectly by measuring the pivoting position of at least one of the two pivotable knives.
In a single-grip harvester head of the type described above, in which the pivotable limbing knives, which are hydraulically driven, press the stem against the upper limbing knife which here forms the fixed support, the press power generated by the pivotable knives must be greater, the heavier the stem. In order to control this, the diameter of the stem is measured continuously and the hydraulic pressure is controlled by means of control and computer equipment in such a manner that the press power increases the greater the diameter of the stem, i.e. the press power exerted on one and the same stem during its feeding is reduced as the stem is being fed. It is essential that the press power is not made too great since an increased press power results in increased friction between the limbing knives and the stem and, thus, in the strain on the feeding rollers increasing as the tree feeding power increases. For this reason it is thus essential that the measuring of the diameter can be carried out in an accurate way. Accurate measuring of the diameter is essential also because the optimisation of the marking of the stem for cross-cutting calculated by the computer equipment is based exactly on the diameter of the stem. Accurate measuring of the diameter requires that the stem during the measuring is correctly placed in the nip between the limbing knives, i.e. abuts properly against the support formed by the upper, fixed limbing knife. However, this is not always the case since the pivotable limbing knives sometimes tend to xe2x80x9cspringxe2x80x9d back to some extent as a result of the weight of the stem. The pivotable limbing knives can also xe2x80x9cspringxe2x80x9d back or be opened as a consequence of a tree, which is essentially horizontally placed in the single-grip harvester head, during processing being stuck in, for example, another tree or the driver of the forest machine on which the head is mounted not being flexible enough in his or her manoeuvring of the head relative to the tree or the stem.
The object of the present invention is to provide a method of controlling the press power generated by the pivotable limbing knives in such a manner that the pivotable limbing knives are safely brought back to a xe2x80x9cclosedxe2x80x9d position as soon as they xe2x80x9cspringxe2x80x9d back.
According to the invention, this object is achieved by a method which is of the type stated by way of introduction and is characterised in
that the diameter values which are measured by degrees during the measuring of the diameter are registered,
that each measured diameter value is compared with a predetermined diameter value being selected from the diameter values of the current stem which are previously registered during the feeding of the stem,
that, if a measured diameter value is found to equal or be greater than this predetermined diameter value, the press power is increased to a corrective power value which is considerably higher than the press power value which, according to the predetermined control diagram, corresponds to this measured diameter value, and
that this corrective power value is maintained until a later measured diameter value falls below a reference value which is somewhat greater than or equals said predetermined diameter value,
after which the press power is once again controlled in accordance with the predetermined control diagram.
Preferably, the corrective power value equals or is greater than the value of the press power which, in accordance with the control diagram, is exerted on the stem where its diameter is at its maximum.
The predetermined diameter value preferably equals the lowest registered diameter value of the current stem.
Preferably, the reference value equals said predetermined diameter value.